Field shaping magnet structure

ABSTRACT

A magnet structure having a pair of spaced pole pieces interconnected by a reluctor circuit comprising an operatively aligned array of soft magnetic members suitably spaced apart by interposed nonmagnetic material to provide between the pole pieces a preferred magnetic scalar potential and an associated magnetic flux distributed as desired over a relatively large volume adjacent the array.

United States Patent 1 1 Dionne 14 1 Dec.2,l975

1 1 FIELD SHAPING MAGNET STRUCTURE [75] Inventor: Norman J. Dionne, Arlington Heights, Mass.

[73] Assignee: Raytheon Company, Lexington,

Mass.

[22] Filed: Nov. 1, 1974 {21] Appl. Nos. 520,074

521 U.S.C1. ..335/2ll;3l5/3.5 511 Int.C1. ..HlF7/02 s FieldofSearch ..335/2i1.210;315/3.5

[561 References Cited UNITED STATES PATENTS 7/1965 Winwood ct a1 3l5/3.5 1/1966 Adler 315/35 3,304,523 2/1967 Jaquen ct a1 315/35 3,373,389 3/1968 Armstrong 335/211 3,404,306 /1968 Johnson 315/ Primary E.raminerHar0ld Broome Attorney, Agent, or Firm-John T. Meaney; Harold A. Murphy; Joseph D. Pannone [57] ABSTRACT A magnet structure having a pair of spaced pole pieces interconnected by a reluctor circuit comprising an operatively aligned array of soft magnetic members suitably spaced apart by interposed nonmagnetic material to provide between the pole pieces a preferred magnetic scalar potential and an associated magnetic flux distributed as desired over a relatively large volume adjacent the array.

10 Claims, 7 Drawing Figures U.S. Patent De. 2, 1975 Sheet 1 of3 3,924,210

U.S. Patent Dec. 2, 1975 Sheet 2 of3 3,924,210

Wm fix I @J fKi 9/ PRIOR ART F/G. '3 PRIOR ART F/G. 4 PRIOR ART U.S. Patent Dec. 2, 1975 Sheet 3 of3 3,924,210

FIELD SHAPING MAGNET STRUCTURE BACKGROUND OF THE INVENTION This invention relates generally to magnet structures and is concerned more particularly with a magnet structure having field shaping means disposed between magnetically coupled pole pieces.

It is well-known that a magnetically coupled pair of spaced north and south poles have established therebetween a magnetic field potential and are linked to one another by associated lines of magnetic flux. Generally, the lines of magnetic flux are crowded quite close together adjacent the respective magnetic poles and indicate correspondingly high field gradients in the regions adjacent the poles. Also, the lines of magnetic flux are spaced relatively wide apart in a central region between the respective poles, thus indicating a substantially lower magnetic field potential in this central region.

If the gap separating the magnetic north and south poles has a transverse dimension which is significantly less than its longitudinal dimension, the pole pieces may be properly shaped to obtain a more uniform flux distribution in the gap. As a result, a correspondingly more uniform field potential is developed across the gap. However, the geometry of the pole pieces can influence the flux distribution only over a limited distance from the respective pole pieces. Consequently, when the transverse dimension of the gap exceeds this limit of pole influence, other means must be found for distributing the magnetic flux between the respective pole pieces as desired.

Therefore, it is advantageous and desirable to provide means for distributing the magnetic flux between an operatively coupled pair of spaced north and south poles so as to obtain a desired field potential therebetween.

SUMMARY OF THE INVENTION Accordingly, this invention provides a magnet structure comprising a pair of spaced pole pieces interconnected by a reluctor circuit having means for shunting magnetic flux along preferred paths between the pole pieces. The circuit includes an operatively aligned array of soft magnetic members disposed between the pole pieces and suitably spaced apart by interposed nonmagnetic material to provide a desired magnetic scalar potential along the array. The magnetic potential decreases monotonically in a nonlinear manner along respective end portions of the array adjacent the pole pieces, and decreases linearly along a central portion of the array. As a result, there is established adjacent the array and at substantial radial distances therefrom a preferred magnetic flux distribution.

A specific embodiment of the invention includes a generally U-shaped magnet structure having respective terminal pole. pieces spaced apart by a relatively long gap therebetween. The pole pieces are joined to one another by a low reluctance member which serves as a flux return path of a reluctor circuit interconnecting the pole pieces. Disposed in the gap between the pole pieces is an array of soft magnetic members comprising a flux distributing portion of the reluctor circuit. The soft magnetic members are suitably spaced from the pole pieces and from one another by respective interposed layers of nonmagnetic material to shunt magnetic flux in the direction of the flux return path in a predetermined manner.

gradient of magnetic In respective end portions of the array, spacing between successive soft magnetic members progressively decreases to provide a magnetic scalar potential which decreases parabolically adjacent the pole pieces. As a result, the associated lines of flux extend outwardly of the pole pieces and axially along the array significantly greater distances than would be normally expected. The array may include respective intermediate portions disposed between the end portions and a central portion of the array. Each of the intermediate portions may comprise a single magnetic member having a relatively longer axial dimension than the other members of the array or may comprise a plurality of closely spaced, soft magnetic members. The intermediate portions provide respective constant values of magnetic scalar potential which serve as transitional regions of the desired gradient. As a result, the lines of flux extending outwardly of the pole pieces are directed along preferred paths which are symmetrically disposed with respect to the array. From either end of the central portion of the array to the midpoint thereof, the spacing between successive soft magnetic members progressively increases to provide a magnetic scalar potential which decreases linearly along the central portion of the array. As a result, the magnetic lines of flux exterior of the magnet structure are maintained rectilinear and parallel with the array in a uniform symmetrical pattern.

An alternative preferred embodiment comprises a magnetic solenoid structure which may be evolved by rotating the above-described, U-shaped magnet structure about an imaginary axis spaced a predetemiined radial distance from the array. Thus, the low reluctance member and the array of soft magnetic members are converted into respective outer and inner coaxial cylinders. At the ends of the solenoid structure, the gaps between the concentric cylinders are bridged by respective annular magnets which are radially polarized in opposite directions relative to one another. The inner coaxial cylinder comprises an array of laminated rings disposed between the annular magnets, alternate rings being made of soft magnetic material and the interposed rings being made of nonmagnetic material. The nonmagnetic rings are provided with respective thickness dimensions suitable for spacing the soft magnetic rings from the annular magnets and from one another as described in connection with the U-shaped magnet structure. Accordingly, the magnetic lines of flux extend radially inward and axially along the cylindrical volume in a uniform symmetrical manner over substantially the entire volume.

Thus, an electron device, such as a proximity focused, image intensifier tube, for example, may be disposed in the central portion of the cylindrical volume such that the magnetic scalar potential decreases linearly from the input end of the tube to the output end thereof. In this manner, the resulting uniformity of the magnetic field will aid in proximity focusing an electron image within the tube onto the output viewing screen thereof.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an axial sectional view of a magnet structure embodying the invention;

FIG. 2 is a schematic view of a prior art magnet struc ture;

FIG. 3 is a graphical view of the component of flux density along a rectilinear line interconnecting the pole pieces and immediately exterior of the magnet structure shown in FIG. 2;

FIG. 4 is a graphical view of the approximate magnetic scalar potential along a rectilinear line interconnecting the pole pieces of the magnet structure shown in FIG. 2;

FIG. 5 is a schematic view of another magnet structure embodying the invention;

FIG. 6 is a graphical view of the component of flux density parallel to the reluctor array and immediately exterior of the magnet structure shown in FIG. 5; and

FIG. 7 is a graphical view of the approximate magnetic scalar potential along the reluctor array between the pole pieces of the magnet structure shown in FIG.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, there is shown in FIG. 2 a U-shaped magnet structure 10 including spaced magnets 11 and 13, respectively, which are made of suitable ferromagnetic material, such as Samarium-cobalt, for example, and are oppositely polarized with respect to one another. Disposed adjacent the magnets 11 and 13 are respective terminal pole pieces 12 and 14 which are spaced apart by a relatively long gap 18 therebetween. The magnets 11 and 13 are joined to one another by a low reluctance member 16 which serves as a return path for lines of magnetic flux extending into gap 18 between the pole pieces 12 and 14.

In practice, it is found that the lines of magnetic flux generally curve arcuately adjacent the pole pieces 12 and 14, but do not necessarily extend across the gap 18 in a rectilinear manner. Some flux lines, such as 21-25 and 26-30, for examples, extend less than half way across the gap 18 and return through the low reluctance member 16. Other flux lines, such as 31-34, for examples, extend across the gap 18 but have arcuate central portions which curve in the direction of the low reluctance member 16. comparatively, few flux lines, such as 35, for example, extend across the gap 18 and have central portions which are substantially rectilinear.

Accordingly, when the parallel component of magnetic flux density B is plotted versus the distance Z across gap 18, as shown in FIG. 3, the resulting curve 36 generally has two crests, 38 and 40, respectively, separated by an intervening trough 42. The crests 38 and 40 indicate that the magnetic flux is concentrated in corresponding regions of gap 18 adjacent the pole pieces, 12 and 14, respectively. Consequently, when the associated approximate magnetic scalar potential Vm is plotted as a function of the distance Z across gap 18, as shown in FIG. 4, the resulting curve generally is similar to the curve 44. Curve 44 indicates that, adjacent the pole piece 12, the magnetic potential decreases rapidly from a maximum positive value and, after passing through an extended region of substantially zero value, rapidly approaches a maximum negative value adjacent the south pole piece 14. As a result, the magnetic field established by the conventional magnet structure 10 shown in FIG. 2 is not very effective at substantial radial distances, as represented by the axial line 46, for example, from the pole pieces 12 and 14.

In FIG. 5, there is shown a magnet structure 50 embodying this invention and comprising spaced magnets 51 and 53, respectively, which aremade of ferromagnetic material, such as Samarium-cobalt, for example, and are suitably coupled to adjacent pole pieces, 52 and 54, respectively. The pole pieces 52 and 54 are spaced apart by an interposed gap 58 which may be equivalent to gap 18, for example. The magnets 51 and 53 are magnetically coupled to one another through suitable flux return means, such as a member 56 similar in design to the member 16 shown in FIG. 2, for example. Preferably, the flux return member 56 is made of a readily available, low reluctance material, such as cold rolled steel, for example, and is provided with a configuration which is symmetrical with respect to the pole pieces 52 and 54. However, the member 56 may be made of any magnetically permeable material and may have any configuration suitable for magnetically coupling the pole pieces 52 and 54 to one another.

Accordingly, the member 56 comprises a flux return portion of a reluctor circuit means for interconnecting the pole pieces 52 and 54, respectively, in a predetermined manner. The circuit also includes a flux distributing array 60 disposed in operative alignment between the pole pieces 52 and 54. Array 60 comprises alternative layers 62 of nonmagnetic material, such as air, for example, and interposed members 64 of soft magnetic material, such as iron, for example. Alternatively, the layers 62 may be made of rigid nonmagnetic material, such as aluminum, for example, and may be secured, as by bonding, for example, to adjacent soft magnetic members 64 of the array 60. The resulting rigid structural array 60 may have extreme end surfaces of respective nonmagnetic layers 62 disposed in interfacing relationship with pole pieces 52 and 54, respectively, and may be suitably secured thereto, as by bonding, for example. The nonmagnetic layers 62 and the interposed soft magnetic members 64 may be provided with respective cross-sectional configurations similar to the pole pieces 52 and 54, respectively, or may be provided with any other suitable cross-sectional configuration.

Adjacent the pole pieces 52 and 54, the array 60 is provided with respective end portions 60a, each including a plurality of soft magnetic members 64 having respective axial thicknesses which are substantially equal to one another. Alternatively, each of the end portions 60a may be provided with soft magnetic members 64 having respective axial thicknesses which differ with respect to one another. However, in either instance, the soft magnetic members 64 in the respective end portions 60a are suitably spaced from the adjacent pole piece and from one another by interposed nonmagnetic layers 62 having steadily decreasing axial thickness dimensions with increasing distance from the adjacent pole piece. As a result, in each end portion 60a, the successive soft magnetic members 64 are spaced progressively closer together as a function of their respective distances from the adjacent pole piece.

The array 60 also may include respective intermedi ate portions 60b which are disposed in operative alignment between respective end portions 60a and a central portion 60c of the array. The intermediate portions 60b may comprise respective unitary soft magnetic members 64 having substantially greater axial thicknesses than the soft magnetic members 64 in the respective end portions 60a. Alternatively, each of the intermediate portions 60b may comprise a plurality of relatively thin soft magnetic members separated by interposed, relatively thin, nonmagnetic layers 62 such that the sum of the parts is comparative in axial length to the thickness of the unitary soft magnetic member 64. The intermediate portions 60b are separated from the adjacent end portions 60a by respective interposed nonmagnetic layers 62 having axial thicknesses which conform to the pattern of progressively decreasing axial thicknesses of the nonmagnetic layers 62 in the respective end portions 60a.

The central portion 60c of array 60 includes a plurality of soft magnetic members 64 having respective axial thicknesses which, preferably are substantially equal to the axial thicknesses of soft magnetic members 64 in the respective end portions 60a for ease of fabrication. Alternatively, the soft magnetic members 64 in the central axial portion 60c may be provided with respective axial thicknesses of any desired dimension. However, in either instance, the soft magnetic members 64 in the central portion 60c are spaced from the adjacent intermediate portions 60b by interposed nonmagnetic layers 62 having steadily increasing axial thickness dimensions with increasing distance from the adjacent intermediate portion 60b toward the midpoint of the central portion 60c. As a result, successive soft magnetic members 64 in going from either end towardthe midpoint of the central portion are spaced progressively farther apart as a function of the distance from the adjacent intermediate portion 60b.

Accordingly, the soft magnetic members 64 of the array 60 conduct the magnetic flux relatively greater distances into the gap 58 from the associated pole pieces 52 and 54, respectively. Also, the soft magnetic members 64 are suitably spaced apart by the interposed nonmagnetic layers 62 to shunt the magnetic flux over to the return member 56 in a manner which will produce a desired magnetic scalar potential profile along the array 60. Thus, magnetic lines of flux 21a-24a and 26a-29a, which are equivalent to flux lines 21-24 and 26-29, respectively, (FIG. 1), extend farther into the gap 56 than would be expected without the array 60. Also, magnetic flux line 30a which is equivalent to joined flux lines 25 and 30, respectively, extends with flux lines 31a, which is equivalent to flux line 31, rectilinearly through the array 60 and between the pole pieces 52 and 54. Furthermore, magnetic flux lines 32a-34a, which are equivalent to flux lines 32-34, respectively, extend between the pole pieces in a substantially rectilinear manner. As a result, the flux lines 32a34a in conjunction with the flux line 35a, which is equivalent to flux line 35, extend axially in a uniformly spaced, rectilinear manner at substantially greater radial distances, as represented by the axial line 46 from the pole pieces than would be the case without the flux distributing array 60 of the reluctor circuit means.

Consequently, when the parallel component of the magnetic flux density B is plotted versus the distance Z across gap 58, and exterior of the reluctor array 60, as shown in FIG. 6, the resulting curve 66 generally has an extended central plateau with respective gradually sloping portions at either end thereof. The curve 66 indicates that the major portion of the magnetic flux is distributed substantially uniformly over a corresponding extended central portion of gap 58 and decreases gradually toward the respective pole pieces 52 and 54.

Therefore, when the associated approximate magnetic scalar potential Vm is plotted as a function of the distance along the array 60, and immediately exterior of the structure 50, the resulting curve 68 generally indicates that the magnetic potential varies monotonically in a nonlinear manner adjacent the respective pole pieces 52 and 54. For example, adjacent the pole pieces, the magnetic scalar potential Vm may vary parabolically, that is it decreases as the square of the distance Z along the array. Alternatively, the soft magnetic members 64 in the respective end portions 60a the array may be suitably spaced by interposed nonmagnetic layers 62 to provide a magnetic scalar potential profile which varies monotonically in any other nonlinear manner, such as exponentially, for example, adjacent the pole pieces 52 and 54, respectively. After passing through respective transitional regions of substantially constant value corresponding to the respective intermediate portions 60b of the array, the magnetic potential varies linearly along a central region corresponding to the central portion 600 of the array. Accordingly, as shown in FIG. 5, uniformly spaced equipotential lines 67 extend through the resulting field of flux and, in the central portion thereof, are substantially perpendicular to the array 60.

In FIG. 1, there is shown a permanent magnet solenoid structure 70, which, in a broad sense, may be considered as the magnet structure 50 rotated about the axial line 46. Thus, the resulting hollow cylindrical structure 70 comprises spaced inner and outer coaxial cylinders, 72 and 74, respectively, which are bridged at respective ends of the structure 70 by annular magnets, 76 and 78, respectively. The magnets 76 and 78 are made of suitable ferromagnetic material, such as samariumcobalt, for example, and are oppositely polarized in the radial direction. Consequently, lines of magnetic flux extend axially between the respective inner peripheries of the magnets 76 and 78, and return through the outer cylinder 74. Accordingly, the outer cylinder 74 is made of suitable low reluctance material, such as cold rolled steel, for example, and comprises the flux return portion of a reluctor circuit means for interconnecting the magnets 76 and 78 in accordance with this invention.

The inner cylinder 72 constitutes the flux distributing portion of the reluctor circuit means and comprises a laminated array 80 of alternate rings 82 made of nonmagnetic material, such as aluminum, for example, and interposed rings 84 made of soft magnetic material, such as iron, for example. The array 80 is provided with respective end portions 80a, each including a plurality of soft magnetic rings 84 suitably spaced from the adjacent annular magnet and from one another by interposed nonmagnetic rings 82, which steadily decrease in axial thickness with increasing distance from the adjacent annular magnet. As a result, successive soft magnetic rings 84 in respective end portions 80a of the array are spaced progressively closer together as a function of their respective distances from the adjacent annular magnet.

The array 80 also may be provided with respective intermediate portions 8012 which are disposed in alignment between respective end portions 80a and a central portion 800 of the array. Preferably, the intermediate portions 80b comprise respective unitary soft magnetic rings 84 of substantially greater axial thicknesses than the soft magnetic rings 84 in the respective end portions 80a of the array. Each of the intermediate portions 80b is separated from the adjacent end portion 80a by an interposed nonmagnetic ring 80b having an axial thickness conforming to the steadily decreasing thicknesses of the nonmagnetic rings 80b in the adjacent end portion 80a of the array.

The central portion 800 of the array 80 comprises a plurality of soft magnetic rings 84 suitably spaced from the adjacent intermediate portion 80b and from one another by interposed nonmagnetic rings 82. In the central portion 800, the nonmagnetic rings 82 are provided with respective increasing axial thicknesses with increasing distance from the adjacent intermediate portion 80b toward the midpoint of the central portion 800. Thus, in going from either of the intermediate portions 80b toward the midpoint of the central portion 80c, successive soft magnetic rings 84 are spaced progressively farther apart as a function of their respective distances from the adjacent intermediate portion 80b.

Accordingly, the magnetic field established within the solenoid structure 70 is similar to the magnetic field of magnet structure 50 (FIG. rotated about the axial line 46. The magnetic lines of flux extending from the respective annular magnets 76 and 78 are directed axially along the array 80 and are shunted over to the flux return cylinder 74 in the manner described in connection with the magnet structure 50. Consequently, a major portion of the magnetic flux is distributed substantially uniformly in a central region of the structure 70 interior of the cylindrical array 80.

The resulting approximate magnetic scalar potential established along the cylindrical array 80 decreases monotonically in a nonlinear manner, such as parabolically, for example, adjacent the annular magnets, 76 and 78, respectively. Along the respective intermediate portions 80b of the array, the magnetic scalar potential remains substantially constant. However, along the extended central portion 80c of the array, the magnetic scalar potential decreases linearly from one end to the other end of the central portion. Thus, transverse equipotential surfaces (not shown) which are similar to equipotential lines 70 shown in FIG. 5, for example, are substantially equally spaced apart along the axis of the structure 70 and, in an extended central portion thereof, are substantially perpendicular to the cylindrical array 80.

Accordingly, an electron device, such as image intensifier tube 90, for example, may be disposed axially within the central portion of the magnetic field generated within the solenoid structure 70. The image intensifier tube 90 may be of the conventional proximity fo- 'cusing type, for example, having an input screen assembly 92 adjacent one end, an output screen assembly 94 adjacent the other end, and a conventional microchannel plate assembly 96 operatively disposed therebetween. In operation, the input screen assembly 92 receives a dim visual image and emits a corresponding electron image which is electrostatically accelerated toward the microchannel plate assembly 96. While passing through aligned apertures in the microchannel plate assembly 96, the electron density of the image is increased correspondingly by secondary emission. The

resulting electron image is electrostatically accelerated cuit having a flux return portion and a flux distributing portion. The flux distributing portion is operatively disposed between the pole pieces and comprises an aligned array of alternate nonmagnetic layers and interposed soft magnetic members. The soft magnetic members are suitably spaced from the pole pieces and from one another by the interposed nonmagnetic layers in a manner which provides a desired magnetic scalar potential profile along the array and between the pole pieces. The desired potential profile decreases monotonically in a nonlinear manner adjacent the pole pieces and decreases linearly in an extended central portion of the space between the pole pieces.

Although the reluctor circuit of this invention has been illustrated with the use of permanent magnets, it may equally well be employed for interconnecting the pole pieces of electromagnets. Furthermore, the reluctor circuit may be employed for interconnecting the pole pieces of other magnet structures than cylindrical solenoid structure shown herein.

From the foregoing, it will be apparent that all of the objectives of this invention have been achieved by the structures shown and described. It also will be apparent, however, that various changes may be made by those skilled in the art without departing from the spirit of the invention as expressed in the appended claims. It is to be understood, therefore, that all matter shown and described is to be interpreted as illustrative and not in a limiting sense.

It is claimed that:

1. A magnet structure comprising:

a pair of spaced magnetic pole pieces operatively disposed in alignment with one another and oppositely polarized in a direction substantially perpendicular to the direction of alignment; and

reluctor circuit means interconnecting the pole pieces for providing a desired magnetic scalar potential profile, the reluctant circuit means including a linear array of alternate nonmagnetic layers and interposed magnetic members disposed between the pole pieces, the magnetic members being spaced progressively varying distances from one another.

2. A magnet structure as set forth in claim 1 wherein the array includes respective end portions, each having a plurality of soft magnetic members spaced progressively closer together as a function of their respective distances from the adjacent pole piece.

3. A magnet structure as set forth in claim 2 wherein the array. also includes a central portion having a plurality of soft magnetic members spaced progressively farther apart as a function of their respective distances from the adjacent end of the central portion toward the midpoint thereof.

4. A magnet structure as set forth in claim 3 wherein the array includes respective intermediate transitional portions disposed in operative alignment between respective end portions and the central portion, each intermediate portion comprising at least one soft magnetic member suitably spaced from the adjacent end portion of the array by an interposed nonmagnetic layer.

5. A magnet structure as set forth in claim 4 wherein the one soft magnetic member has a greater axial thickness than the thicknesses of the soft magnetic members in the adjacent endportion of the array.

6. A solenoid magnet structure comprising:

a pair of axially aligned annular magnets oppositely polarized with respect to one another in the radial direction and operatively spaced apart for magnetically coupling to one another by an associated axial magnetic field of flux;

a flux return cylinder made of magnetically permeable material and having respective end portions suitably coupled to outer peripheral portions of the annular magnets; and

a flux distributing cylinder operatively disposed between respective inner peripheral portions of the annular magnets and comprising an aligned array of laminated rings, alternate rings being made of nonmagnetic material and the interposed rings being made of soft magnetic material.

7. A solenoid magnet structure as set forth in claim 6 wherein the array includes respective end portions, each having a plurality of soft magnetic rings spaced 10 progressively closer together as a function of their respective distances from the adjacent magnet.

8. A solenoid magnet structure as set forth in claim 7 wherein the array also includes a central portion having a plurality of soft magnetic rings spaced progressively farther apart as a function of their respective distances from the adjacent end of the central portion toward the midpoint thereof.

9. A solenoid magnet structure as set forth in claim 8 wherein the array also includes respective intermediate transitional portions operatively disposed between respective end portions and the central portion of the array, each intermediate portion comprising at least one soft magnetic ring suitably spaced from the adjacent end portion by an interposed nonmagnetic ring.

10. A solenoid magnet structure as set forth in claim 9 wherein the one soft magnetic ring has a greater axial thickness than the thicknesses of the soft magnetic rings in the adjacent end portion of the array. 

1. A magnet structure comprising: a pair of spaced magnetic pole pieces operatively disposed in alignment with one another and oppositely polarized in a direction substantially perpendicular to the direction of alignment; and reluctor circuit means interconnecting the pole pieces for providing a desired magnetic scalar potential profile, the reluctant circuit means including a linear array of alternate nonmagnetic layers and interposed magnetic members disposed between the pole pieces, the magnetic members being spaced progressively varying distances from one another.
 2. A magnet structure as set forth in claim 1 wherein the array includes respective end portions, each having a plurality of sOft magnetic members spaced progressively closer together as a function of their respective distances from the adjacent pole piece.
 3. A magnet structure as set forth in claim 2 wherein the array also includes a central portion having a plurality of soft magnetic members spaced progressively farther apart as a function of their respective distances from the adjacent end of the central portion toward the midpoint thereof.
 4. A magnet structure as set forth in claim 3 wherein the array includes respective intermediate transitional portions disposed in operative alignment between respective end portions and the central portion, each intermediate portion comprising at least one soft magnetic member suitably spaced from the adjacent end portion of the array by an interposed nonmagnetic layer.
 5. A magnet structure as set forth in claim 4 wherein the one soft magnetic member has a greater axial thickness than the thicknesses of the soft magnetic members in the adjacent end portion of the array.
 6. A solenoid magnet structure comprising: a pair of axially aligned annular magnets oppositely polarized with respect to one another in the radial direction and operatively spaced apart for magnetically coupling to one another by an associated axial magnetic field of flux; a flux return cylinder made of magnetically permeable material and having respective end portions suitably coupled to outer peripheral portions of the annular magnets; and a flux distributing cylinder operatively disposed between respective inner peripheral portions of the annular magnets and comprising an aligned array of laminated rings, alternate rings being made of nonmagnetic material and the interposed rings being made of soft magnetic material.
 7. A solenoid magnet structure as set forth in claim 6 wherein the array includes respective end portions, each having a plurality of soft magnetic rings spaced progressively closer together as a function of their respective distances from the adjacent magnet.
 8. A solenoid magnet structure as set forth in claim 7 wherein the array also includes a central portion having a plurality of soft magnetic rings spaced progressively farther apart as a function of their respective distances from the adjacent end of the central portion toward the midpoint thereof.
 9. A solenoid magnet structure as set forth in claim 8 wherein the array also includes respective intermediate transitional portions operatively disposed between respective end portions and the central portion of the array, each intermediate portion comprising at least one soft magnetic ring suitably spaced from the adjacent end portion by an interposed nonmagnetic ring.
 10. A solenoid magnet structure as set forth in claim 9 wherein the one soft magnetic ring has a greater axial thickness than the thicknesses of the soft magnetic rings in the adjacent end portion of the array. 